1. Field of the Invention
The present invention relates to a receiving apparatus for receiving a spread spectrum signal, specifically a multi-code spread spectrum signal.
2. Related Background Art
A spread spectrum communication system employing a direct sequence spread method is a method of generating, from a base-band signal of a digital signal to be transmitted, a base-band signal having a significantly wide band width with respect to original data. This is accomplished by using a spread code sequence, such as a pseudo noise code (a PN code). Furthermore, modulation, such as PSK (Phase Shift Keying) or FSK (Frequency Shift Keying), is performed to convert the base-band signal into an RF (Radio Frequency) signal so as to transmit the RF signal. A receiver unit uses the same spread code as that used in a transmitter unit to perform a despread operation to perform correlation with the received signal so as to convert the received signal into a narrow-band signal having a band width that corresponds to the original data. Then, a normal data demodulation is performed so that the original data is reproduced.
Since the spread spectrum communication system employs a significantly wide transmission band width with respect to the information band width as described above, the foregoing system cannot realize an unsatisfactorily low transmission speed as compared with a typical narrow band width modulation system if the transmission band width is within a certain condition. To overcome the foregoing problem, a multi-code method has been employed. The foregoing method includes the steps of converting a high-speed information signal into low-speed parallel data, spread-modulating the parallel data in different spread code sequences so as to add data, and converting the data into an RF signal that is then transmitted, so that high speed data transmission is realized under a predetermined condition of the transmission band width without deterioration in the spread rate in the spread modulation.
FIG. 31 shows the structure of a transmission mechanism adapted to the foregoing method. Supplied data is converted into n parallel data items by a serial-parallel converter 301. Each converted data is, by a multiplier group consisting of n multipliers 302-1 to 302-n, multiplied by n different spread code outputs from a spread-code generator 303 so as to be converted into wide band spread signals over n channels. Then, the outputs from the respective multipliers are added by an adder 304 so as to be provided to a high-frequency transmitter stage 305. The added wide spread base-band signals are, by the high-frequency transmitter stage 305, converted into a transmission frequency signal having an appropriate central frequency so as to be transmitted by a transmission antenna 306.
FIG. 32 shows the structure of a receiver. The signal received by an antenna 401 is appropriately filtered and amplified by a high-frequency signal processor 402 so as to be converted into a signal having an intermediate frequency. The intermediate-frequency signal is distributed to n channels connected in parallel to correspond to the spread codes. In each channel, the correlation of the input signal with outputs from spread code generator group 404-1 to 404-n is detected in correlator group 403-1 to 403-n so as to be despread, the spread code generator group 404-1 to 404-n corresponding to the channels of the correlator group 403-1 to 403-n. Synchronization of the despread signal is established at each channel in synchronizing circuit group 405-1 to 405-n so that the code phases and clocks of the spread code generators are made to coincide with one another. The despread signals are also demodulated in demodulator group 406-1 to 406-n so that data is reproduced. Then, reproduced data is converted into serial data in serializer 407 so that the original information is reproduced.
However, since the correlator at each demodulation channel acts as the intermediate frequency stage, the conventional structure suffers from a problem in that the size of the circuit cannot be reduced.